1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium of a single plate type comprising a recording layer composed of a dye, having a high reflectivity, capable of recording a large volume of information and to a process for producing the medium.
2. Description of the Related Art
Writable optical recording mediums of a single plate type playable by a commercially available compact disc (hereinafter called "CD") which comprises a recording layer composed of a dye, a metal reflective layer overlying the recording layer for enhancing reflectivity and a protective layer overlying the reflective layer are proposed, for example, in Optical Data Storage 1989 Technical Digest Series Vol. 1, 45 (1989), and EP-353393.
In these optical recording mediums, a protective layer is provided on a reflective layer in order to protect a recording layer and the reflective layer. As the protective layer, a hard resin, so-called "hard coat", is usually used to prevent the damage of the recording layer and the reflective layer.
Further, additional functions of the protective layer are the prevention of peeling of the recording layer and the reflective layer, inhibition of oxidative deterioration of the recording layer and the reflective layer, and thereby serving to improve the durability. Therefore, the adhesion between the protective layer and the reflective layer is required to be strong.
However, in the case of a medium using a dye in a recording layer, the recording layer is usually formed by spin-coating. An optical disc substrate has a groove, and when a recording layer is formed by spin-coating, the recording layer is formed on the whole surface of the substrate ranging from the further inside of the groove at the inner peripheral portion of the substrate to the outermost edge of the substrate.
Taking into consideration the adhesion between the protective layer and the reflective layer, it is demanded to remove the recording layer at the inner peripheral and outer peripheral portions of the substrate and form a protective layer extending over the peripheries of the recording layer and the reflective layer as proposed by Japanese Patent Application Laid-Open Nos. Hei 2-183442 and 2-236833.
In order to form the protective layer extending over both the recording layer and the reflective layer as shown, for example, in FIG. 4, it is necessary to provide a portion where the substrate is exposed at each of the inside of the innermost periphery and the outside of the outermost periphery (outermost edge) of the recording layer.
For the purpose of forming such a structure, heretofore, the recording layer formed at each of the outer and the inner peripheral portions has been removed. For example, the recording layer at the outer peripheral portion of the substrate can be easily removed by dropping a solvent to the peripheral portion while rotating the substrate.
On the contrary, the recording layer at the inner peripheral portion can not be removed by the same method as that used for removing the recording layer at the outer peripheral portion, but is removed only by, for example, wiping the recording layer away with a solvent or burning the recording layer with laser as disclosed in Japanese Patent Application Laid-Open No. Hei 2-183442.
However, according to the present inventors' investigation, those methods are complicated and of less productivity, and the recording layer can not be completely removed. And we have found that the adhesion of the protective layer is not sufficient, and therefore when the medium is allowed to stand under high temperature and high humidity conditions for a long period of time, the peeling strength at the inner peripheral portion is poor so that peeling occurs at the inner peripheral portion.
Alternatively, a recording layer at the inner peripheral portion can be removed as shown in Japanese Patent Application Laid-Open No. Hei 2-236833. That is, a reflective layer is formed on a recording layer in such a way that the recording layer protrudes from the inner and the outer edges of the reflective layer and the recording layer protruding the inner and outer edges of the reflective layer is washed away with a solvent.
We have found that, according to this method, when the reflective layer is washed with a solvent, it is damaged and thereby, the reflective layer is peeled off and noise of the resulting medium increases, because the reflective layer is a very thin metal film.
This method also includes a complicated solvent washing step and further, the productivity is poor.
The present inventors have variously researched so as to develop a method for producing easily and at a high productivity an optical recording medium of a single plate type such that the adhesion of a protective layer to a reflective layer is excellent and no peeling occurs at the inner peripheral portion and the outer peripheral portion when subjected to a tape peeling test after the medium is stood under high temperature and high humidity conditions for a long period of time.
The present inventors have noticed an important-fact that the remaining stress at the protective layer etc. is different depending on the positions of the layer, that is, the inner peripheral portion and the outer peripheral portion, and the remaining stress at the inner peripheral portion is much smaller than that at the outer peripheral portion. The present inventors have found that the above-mentioned disadvantages can be solved by (1) forming a reflective layer extending over the recording layer and further forming a protective layer extending over the reflective layer at the outer peripheral portion of the substrate and (2) forming a protective layer extending over both the recording layer and the reflective layer at the inner peripheral portion, that is, it is not necessary that the reflective layer extends over the recording layer at the inner peripheral portion as shown in FIG. 3.